<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>画三角形</title>
  <style>
    .app {
      width: 0;
      height: 0;
      border: 100px solid;
      border-color: black transparent  transparent;
    }
  </style>
</head>
<body>
  <div class="app"></div>
</body>
</html>